This invention relates to rubber springs utilizing cylinders and rollers made of rubber and plastic composites which are composed of a generally cylindrical body of rubber in compression that surrounds a metal sleeve while its exterior surface is connected to an outer plastic shell that holds such rubber in compression.
In the vulcanization of rubber of cylindrical torsion and linear shear springs, the rubber takes its approximate final shape at the high temperature required for vulcanization and then contracts on cooling. In order to minimize tension on the external shell of a torsion spring it has been the practice to form the shell in halves and then to move the two halves of the shell toward one another in the mounting or have the two shell halves split but mounted on the rubber cylinder and have the split between the halves filled with the molded rubber. These methods seek to eliminate tension in the rubber. By filling the gap between the two halves with the molded rubber some of the concentration of stress at the gap is eliminated.
The present invention eliminates the stress in an unusual manner while simultaneously insuring a superior and stronger product. The outer shell halves are made of ultra high molecular weight polyethylene which produces a marked thermal shrinkage between the outer shell halves and the intermediately located rubber molded sleeve such that the outer shell halves fuse at their mating ends to form a perfectly round shell onto a perfectly round rubber cylinder applying desirable compressive forces upon the rubber which is achieved by the vulcanization process and places the rubber in compression in the finished product. In this manner the thermal stresses within the rubber are relieved and introduces compressive forces within the rubber which is desirable in that it provides good flex life in the rubber. Prior to this assembly, the typical arrangement of the outer shell halves as discussed above had the centers of the shell radius spaced apart from the center of the inner shaft or the center of the rubber cylinder wherein reliance was made on the fact that on shrinkage of the rubber cylinder, the spaced centers of the two outer shell halves would move relatively close to the center of the rubber cylinder. With the present use of the ultra high molecular weight materials for the outer shell halves, it is the shrinkage of the shell halves in cooperation with the shrinkage of the rubber cylinder that provides a common center point in a more facile manner due to their inherent shrinkage rates while introducing desirable compressive forces within the rubber. In addition to these desirable features of using this combination, the outer shell can be machined to closer tolerances. Further, if necessary to provide a friction fit, the entire curved assembly can be cooled to a very low temperature and thence inserted into the bore of an outer housing such that with the resulting expansion on warming up to ambient temperature of the torsion spring assembly an excellent friction fit is obtained. In a similar manner the present invention provides a facile means for manufacturing idler rollers which are particularly economical and simple to manufacture yet exhibiting the qualities of abrasion and wear resistance. These qualities are combined with the ability of the idler rollers to withstand shock loading. The present invention further utilizes the properties of this invention in a shear spring by applying the compressive forces of the shrinking outer ultra high molecular weight polyethylene shell or ring which is bonded to a rubber ring which in turn is bonded to a metal sleeve. The coefficient of thermal expansion of the ultra high molecular weight polyethylene is approximately 3.6 times the amount of shrinkage of the rubber ring, thus providing a great compressive force on the rubber to achieve a good flex life in its action as an axial shear spring. The interaction of the shrinkage of the ultra high molecular weight polyethylene provides a significant contribution to this shear spring combination.